The present invention relates to a signal read circuit for reading out an output from a photoelectric conversion element which is a solid-state image sensor such as an image pickup device or a MOS image sensor and, more particularly, to a signal read circuit for reading out an output from a photodiode array which monitors light transmitted through an optical fiber by wavelength division multiplexing by demultiplexing the light into a plurality of components.
The present inventors have proposed a signal read circuit for reading out an output signal from a photodiode array. This signal read circuit is described in Japanese Patent Laid-Open No. 10-336526. Recently, WDM (Wavelength Division Multiplexing) optical communication has attracted attention, and the development of an apparatus which demultiplexes an output from an optical fiber into different wavelengths and monitors each wavelength component is expected.
When the individual wavelength components are input to the photodiode array, the output from the optical fiber can be monitored at each wavelength. Although the signal read circuit achieves its superior characteristics in an application like this, the characteristics are unsatisfactory and required to be further improved. Especially when in actual use the temperature of the ambient environment of the sensor changes, the output offset level fluctuates along with this temperature change. This deteriorates the absolute output accuracy. The present invention relates to improvements of this prior art, and has as its object to provide a signal read circuit capable of lowering the noise level of an output signal from a photoelectric conversion element.
To achieve the above object, a signal read circuit according to the present invention is a signal read circuit comprising a first circuit row having a charge amplifier connected to a photoelectric conversion element and a CDS circuit for performing correlated double sampling for an output from the charge amplifier, characterized by comprising a second circuit row having the same arrangement as the first circuit row and connected in parallel with the first circuit row, wherein two input terminals of the charge amplifier in the second circuit row are open, and output terminals of the first and second circuit rows are connected to a subsequent circuit such that an offset level generated in the first circuit row decreases.
In this signal read circuit, the subsequent circuit calculates the difference between the circuit rows, thereby removing offset level variations particularly generated in the two circuits when a temperature changes. This improves the offset level uniformity of an output signal from the photoelectric conversion element. Especially when this photoelectric conversion element is made of a compound semiconductor, a dark current of the photoelectric conversion element significantly differs from one element to another. When a plurality of photoelectric conversion elements are used, therefore, the offset level significantly differs from one circuit row to another. In a case like this, the difference configuration described above effectively functions to reduce variations in this offset level.
For example, when the signal read circuit further comprises a differential output circuit for outputting a difference between outputs from the first and second circuit rows, offset variations generated in these two circuit rows can be removed.
This differential output circuit can comprise a differential amplifier circuit which has an operational amplifier having an inverting input terminal, non-inverting input terminal, and output terminal, first and second resistors interposed between the outputs of the first and second circuit rows and the inverting and non-inverting input terminals, respectively, and a third resistor interposed between the output terminal and the inverting input terminal.
Also, the differential output circuit preferably comprises selecting means for selectively connecting the outputs from the first or second circuit row to one terminal of a capacitor, and a switch which, when the selecting means selects one of the first and second circuit rows to one terminal of the capacitor, connects the other terminal of the capacitor to a fixed potential, and, when the selecting means connects the other one of the first and second circuit rows to one terminal of the capacitor, disconnects the other terminal of the capacitor from the fixed potential. In this arrangement, fluctuations in the output signal level caused by temperature changes can be suppressed, since the temperature dependence of a capacitor is much lower than that of a resistor.
Instead of the differential output circuit, the signal read circuit can further comprise calculating means for calculating the difference between the outputs from the first and second circuit rows. This calculating means can be a computer.
The first and second circuit rows are preferably formed on the same semiconductor substrate. This arrangement can achieve circuit characteristics more uniform than when these circuits are formed on different substrates. When the differences between the outputs from these circuits are calculated, therefore, the offset variations can be further reduced.
In the signal read circuit, a third circuit row having an open input terminal to which no input signal is applied, and having the same configuration as that of the first circuit row, is preferably connected in parallel with the first circuit row on the semiconductor substrate. In this case, an output from this third circuit row having similar output characteristics can be used.
In particular, the first circuit row is desirably placed between the second and third circuit rows. In this case, the output from the first circuit row on the semiconductor substrate can be regarded as substantially equal to the average value of the outputs from the second and third circuit rows. Accordingly, the influence of the formation positions of these circuit rows can be suppressed. That is, the signal read circuit according to the present invention preferably comprises a differential output circuit for outputting a difference between an average value of the outputs from the second and third circuit rows and the output from the first circuit row. This differential output circuit need not be formed on the same semiconductor substrate. Additionally, the difference can also be calculated by inputting these outputs to a calculating means such as a computer.